When using a digital multimeter to measure resistance in automotive circuits, which safety practice is essential?

• A. Disconnect power before measuring resistance
• B. Always power the circuit on before testing
• C. Use bare, uninsulated leads
• D. Wear gloves only if the engine is hot

Answer: (A)

Measuring resistance with a digital multimeter relies on the meter sending a small test current through the component and measuring the resulting voltage drop. If the circuit is powered, that power can interact with the meter’s test current, giving an inaccurate reading and potentially harming components, blowing fuses, or causing arcing. Turning off the power (disconnecting the battery or removing the power source) isolates the circuit so the meter can apply its test current safely and obtain a true resistance value. In automotive work, batteries and controllers can supply significant current and there may be stored energy in capacitors, so de-energizing the circuit protects you and the equipment while you measure.